Civil Engineering Reference
In-Depth Information
50
7
Water to cement ratio = 0.43
Air content: 4 to 5%
Cement: Type I,
310 kg/m
3
(517 lb/yd
3
)
Mix data:
Water to cement ratio = 0.43, Type I cement
= 0.45, Type III cement
Air content: 4 to 5%
Cement content: 310 kg/m
3
(517 lb/yd
3
)
6
40
6
40
5
5
30
4
30
4
3
20
3
20
Type III, mixed and cured
at 23
F)
Type I, mixed and cured
at 23
°
C (73
°
2
2
F)
Type III, mixed and cured
at 4
°
C (73
°
10
10
1
1
F)
Type I, mixed and cured
at 4
°
C (40
°
°
C (40
°
F)
0
0
0
1
1
3
7
28
90
365
3
7
28
Age of test, days
Age at test, days
Fig. 14-7. Early-age compressive-strength relationships for
Type I and Type III portland cement concretes mixed and
cured at 4°C (40°F) compared to 23°C (73°F) (
Klieger 1958
).
Fig. 14-5. Effect of temperature conditions on the strength
development of concrete. Concrete for the lower curve was
cast at 4°C (40°F) and placed immediately in a curing room
at -4°C (25°F). Both concretes received 100% relative-
humidity curing for first 28 days followed by 50% relative-
humidity curing (
Klieger 1958
).
Higher-early strengths can be achieved through use of
Type III high-early-strength cement as illustrated in Fig.
14-7. Principal advantages occur during the first 7 days. At
a 4°C (40°F) curing temperature, the advantages of Type
III cement are more pronounced and persist longer than at
the higher temperature.
140
Mix data:
Water to cement ratio = 0.43
Air content: 4 to 5%
Cement: Type I,
310 kg/m
3
(517 lb/yd
3
)
120
HEAT OF HYDRATION
Concrete generates heat during hardening as a result of
the chemical process by which cement reacts with water to
form a hard, stable paste. The heat generated is called heat
of hydration; it varies in amount and rate for different
cements. Dimensions of the concrete placement, ambient
air temperature, initial concrete temperature, water-
cement ratio, admixtures, and the composition, fineness,
and amount of cementitious material all affect heat gener-
ation and buildup.
Heat of hydration is useful in winter concreting as it
contributes to the heat needed to provide a satisfactory
curing temperature; often without other temporary heat
sources, particularly in more massive elements.
Concrete must be delivered at the proper temperature
and account must be taken of the temperature of forms,
reinforcing steel, the ground, or other concrete on which
the fresh concrete is cast. Concrete should not be cast on
frozen concrete or on frozen ground.
Fig. 14-8 shows a concrete pedestal being covered
with a tarpaulin just after the concrete was placed.
Tarpaulins and insulated blankets are often necessary to
retain the heat of hydration more efficiently and keep the
concrete as warm as possible. Thermometer readings of
100
80
60
40
Curing:
Specimens cast and moist-cured
at temperature indicated for first
28 days. All moist-cured at
23
20
°
C (73
°
F) thereafter.
0
1
3
7
28
90
365
Age at test, days
Fig. 14-6. Effect of low temperatures on concrete compres-
sive strength at various ages. Note that for this particular
mixture made with Type I cement, the best temperature for
long-term strength (1 year) was 13°C (55°F) (
Klieger 1958
).
